The present invention relates generally to an apparatus and method for detecting a ground fault. More specifically, it relates to an apparatus and method for detecting a ground fault while providing welding, plasma cutting, and/or induction heating power.
Welding, plasma cutting, and induction heating power supplies are well known. Typically, such power supplies do not have a ground fault interrupt circuit, but rather include reduced open circuit output voltages, low current, high frequency starting circuits, and cautionary labels. However, it is difficult to provide a reduced output voltage for induction heating.
Ground fault interrupting circuits are known outside the welding, plasma cutting, and induction heating industry, but using known ground fault interrupting circuits in the welding, plasma cutting, and induction heating industry is difficult because of the nature of the power supplies and the environments in which they are used.
FIG. 11 shows a common type of prior art ground fault protection circuit used in various locations, including homes. In this circuit, two floating leads 800 and 801 of the power supply pass through the center of a current transformer 802. The secondary winding of current transformer 802 is typically connected to a relay driver circuit 803. The relay 804 in turn is connected to shut down the power supply in the event of a ground fault.
When no ground fault (e.g. no unintended current path) is present, all of the current flowing through first lead 800 (e.g. power lead) returns through second lead 801 (e.g. return lead) and the total net current flowing through current transformer 802 is zero. No voltage is established in the secondary winding of current transformer 802 during normal operation and relay 804 is not tripped. However, if a ground fault is present (an alternative current path develops for the current flowing out of the power supply to ground), at least some of the current flowing in power lead 800 will flow through the fault path to ground. The net current flowing through current transformer 802 is therefore no longer zero since a portion of the return current is escaping to ground. Thus a signal is established in the secondary of current transformer 802. If enough current is diverted to ground through the ground fault, the signal in the secondary will drive relay 804 to trip thereby shutting down the power supply.
This type of ground fault protection device is sensitive, but susceptible to high frequency noise. The problem is exacerbated when the load is inductive, such as a welder, plasma cutter, or an induction heater. This type of device is intended for use with 50-60 Hz line voltages typically found in the home and power supplies having a high frequency stage, such as welders, plasma cutters, or induction heaters, are more likely to create high frequency noise that limits the usefulness of such ground fault protection devices.
A second type of prior art ground fault protection circuit in common use is shown in FIG. 12. One end of the primary winding 903 of a transformer 901 is connected to a floating output 902 of the power supply 900. The other end of primary winding 903 is connected to ground. One end of the secondary winding 904 of transformer 901 is connected to a voltage source 905. The other end of secondary winding 904 is connected to one end of a relay coil 906. The other end of relay coil 906 is connected back to voltage source 905 to complete a detection circuit. A mechanical push button reset 907 is also typically included in the circuit.
Absent a ground fault, there is no complete circuit path to ground. In the event of a ground fault between floating power supply output 902 and ground, however, a path to ground is formed and current will flow in primary winding 903. The current flowing in secondary winding 904 increases as well. If the increase is large enough, a relay will be tripped and the power supply will shut down. Relay coil 906 has an appropriate trip threshold. The normally open contacts 908 across secondary winding 904 are also closed which shorts out secondary winding 904. This latches the relay and power supply 900 remains shut down until reset 909 is triggered. This prior art circuit uses relays which typically require a current of 20 to 100 mA or more on the primary side to trip relay coil 906.
Accordingly, it is desirable to have a welding, plasma cutting, and/or induction heating power supply with a ground fault protection circuit that has a low current threshold but is not adversely susceptible to the high frequency noise typically generated by welding, plasma cutting, and induction heating power supplies.
The present invention relates generally to an apparatus and method for detecting a ground fault. More specifically, it relates to an apparatus and method for detecting a ground fault while providing welding, plasma cutting, and/or induction heating power.
According to a first aspect of the invention, a welding/plasma/induction heating power supply having a ground fault interrupt circuit includes a power supply, a transformer, an impedance, and a comparator. The power supply has a floating output. The transformer includes a first winding in electrical communication between the floating output and a ground. The transformer also includes a second winding in electrical communication with a voltage source. The impedance is in electrical communication with the voltage source such that the impedance in combination with the second winding form a voltage divider. The comparator is connected to receive a sense signal responsive to a voltage across the impedance. The comparator provides an interrupt signal indicative of the existence of a ground fault when the sense signal crosses a threshold.
According to a second aspect of the invention, a method of supplying welding/plasma/induction heating power, including sensing for a ground fault includes providing a floating welding/plasma/induction heating output, dividing a voltage from a voltage source between a first transformer winding and an impedance, wherein a sensed voltage across the impedance is responsive to a leakage impedance across the floating welding/plasma/induction heating output to a ground, comparing a sensed signal responsive to the sensed voltage to a threshold, and providing an interrupt signal indicative of the existence of a ground fault when the sensed signal crosses the threshold.
According to a third aspect of the invention, a welding/plasma/induction heating power supply having a ground fault interrupt circuit includes a power supply, a first transformer, a second transformer, and a detection stage. The power supply includes a floating output. The first transformer has a first winding in electrical communication with the floating output and a second winding in electrical communication with a voltage source. The second transformer is in electrical communication with the voltage source such that the second transformer in combination with the second winding form a voltage divider. The detection stage is connected to receive a sense signal responsive to the voltage across the second transformer. The detection stage provides an interrupt signal indicative of the existence of a ground fault when the sense signal crosses a threshold level.
In one embodiment, the welding/plasma/induction heating power supply also includes a filter in electrical communication with the impedance for filtering out high frequency signals and noise.
In a second embodiment, the threshold is responsive to the voltage source. In an alternative embodiment, the threshold is responsive to a utility line input signal to the power supply. In yet another embodiment, the voltage source is responsive to a utility line input signal to the power supply.
In another embodiment, the impedance is a transformer winding. In alternative embodiments, the impedance is a capacitor or an inductor.
In yet another embodiment, substantially no current flows through the impedance in the absence of a ground fault.
In another embodiment, the method includes dividing the voltage between the first transformer winding and a second transformer winding, wherein the sensed voltage is across the second transformer winding.
In alternative embodiments, the method includes dividing the voltage between the first transformer winding and either a capacitor or an inductor, wherein the sensed voltage is across the capacitor or the inductor.